1. Field of the Invention
The present invention relates to a cold-rolled steel sheet which is superior both in deep drawability and internal anisotropy or stiffness and which is suitable for use as the material of automotive panels and other parts. The invention also is concerned with a method of producing such a cold-rolled steel sheet.
2. Description of the Prior Art
Cold-rolled steel sheets to be used as materials of automotive panels are required to have superior deep drawability. To this end, the cold-rolled steel sheet is required to have a high Lankford value (referred to as r-value) and a high ductility (El).
Hitherto, assembly of an automobile has been conducted by preparing a large number of pressed parts and assembling these parts by spot welding. A current trend, however, is to integrate some of these parts into one piece of a large size, so as to reduce the number of parts and the number of welding spots, in order to improve the product quality while reducing the cost.
For instance, an oil pan of an automobile which has a very complicated form is usually fabricated by welding a plurality of segments. In recent years, however, there is an increasing demand by automotive manufacturers for integral formation of the oil pan. On the other hand, the designs of automobiles are sophisticated and complicated, in order to cope with the demand for diversification of the needs. Consequently, there exist many complicated parts which cannot be formed from conventional steel sheets. Thus, cold-rolled steels having much more superior deep drawability than known steel sheets are being demanded.
Internal anisotropy of the Lankford value (r-value) is a significant factor for successfully carrying out deep drawing. More specifically, the internal anisotropy of the material has to meet the condition of r.sub.max -r.sub.min .ltoreq.0.5, where r.sub.max and r.sub.min respectively represent the maximum and minimum values of the Lankford value.
Another significant factor for integral formation is the stiffness of the material. More specifically, the cold-rolled steel sheet is required to have a Young's modulus of about 23000 kgf/mm.sup.2 as a mean value.
Hitherto, various methods have been proposed for improving deep drawability. For instance, Japanese Examined Patent Publication Nos. 44-17268, 44-17269 and 44-17270 disclose methods in which a low-carbon rimmed steel is subjected to two stages of cold rolling and annealing, so that the r-value is increased to 2.18. This level of r-value, however, cannot provide sufficient deep drawability any more. A publication "IRON AND STEEL (1971), 5280" discloses that a steel sheet for ultra-deep drawing having an r-value of 3.1 can be obtained by preparing a steel having a composition containing C: 0.008 wt %, Mn: 0.31 wt %, P: 0.012 wt %, S: 0.015 wt %, N: 0.0057 wt %, Al : 0.036 wt % and Ti: 0.20 wt %, subjecting the steel to a primary rolling at a rolling reduction of 50%, an intermediate annealing at 800.degree. C. for 10 hours, a secondary rolling at rolling ratio of 80% and a final annealing at 800.degree. C. for 10 hours. This method, however, cannot provide sheet thickness of ordinarily used sheets which is 0.6 mm or greater, because the total cold rolling reduction is as large as 90%. In addition, this publication does nor mention not suggest any anisotropy of the r-value and the young's modulus.
Proposals have been made also for production of cold-rolled steel sheets having superior stiffness. For instance, Japanese Unexamined Patent Publication No. 57-81361 discloses a method in which a cold-rolled steel sheet having a superior stiffness of 23020 kgf/mm.sup.2 in terms of Young's modulus (mean value) is obtained by preparing a steel of a composition containing C: 0.002 wt %, Si: 0.02 wt %, Mn: 0.42 wt %, P: 0.08 wt %, S: 0.011 wt %, N: 0.0045 wt %, Al: 0.03 wt % and B: 0.0052 wt %, cold rolling the steel and then subjecting the steel to continuous annealing at 850.degree. C. for 1 minute. This publication also fails to mention any r-value of the material and, hence, no specific consideration is given to deep drawability.